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I've replaced the plugs in my '09 heads, with no issues at all. But to be fair, they were only in there for 500 miles. The stock ones were in there for 8k miles and removed with no issues. Heads were cold, too.
I used the NGK-equivalent of the 1-heat range colder FRPP plug, NGK 7554, ZNAR7AIX.

Were your COP boots brown or black?

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I have COPs that are from a '07, with the part number that starts with a "3". They are NOT the proper ones that come with the '08+ engines, which begin with a "8" I think. (The '09 engine I obtained did not have the COPs on them, so i had to find some.)
I've not had any issues using them, but would like to have the proper boots; the spring contacts the plug ok, but the boot itself is a little loose on the plug.

How often do we have to change plugs? I have 2005 Gt with 50000, do I have to replace them? If yes, wchich brand is the best?

You will want the Champion 7989 plugs. Ford says the stockers are good for 100K, but I have never run spark plugs that long. You also have the risk of breaking some due to carbon buildup and the stock 2 piece plug problems. There are many threads on this issue. The reason why I want to know what heads I have because I don't know if my car has the old head/plug style.

There are two heads. They are marked on the driver's side valve cover with a giant white sticker with a bunch of numbers and the letters "AA" or "AB". I think the AA's are the two piece **** design, and the AB's are the revised. My 08 was build in mid 2007 and has the AA heads.

There are two heads. They are marked on the driver's side valve cover with a giant white sticker with a bunch of numbers and the letters "AA" or "AB". I think the AA's are the two piece **** design, and the AB's are the revised. My 08 was build in mid 2007 and has the AA heads.

Thanks you so much Sir!

My car has the AB on the sticker and I am so relieved. I don't have to worry about the two piece plug failure now.

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